This invention relates generally to an apparatus for detecting when an electrical load has a change in its impedance and for disconnecting the load from a power source to which it is connected when the change is sufficiently large. More particularly, but not by way of limitation, the invention relates to an apparatus for detecting a fault in an electric blanket heating element and for causing a fuse to open in response to the fault being detected so that the electric blanket heating element will be deactivated, thereby precluding the possibility of a fire resulting from the defective electric blanket heating element.
An electrical energy source is used to energize a device for the purpose of having the device perform some function. The device which is energized is referred to as an electrical load which is electrically quantifiable and identifiable as impedance. The impedance of the load is important because it determines the amount of energy which is drawn from the energy source to energize the load device. Because the impedance of the load can vary due to the utilization and aging of the device, it is important to monitor the load to determine when the impedance significantly changes so that the load can be disconnected from the energy source whereby neither the device is damaged nor a user of the device is injured through the continued use of the device which has become defective because of the changed impedance.
By way of an example, an electric blanket having a heating element constructed of an electrical conductor is a device which provides a load to the 120-V.sub.ac energy source provided by an ordinary home electrical outlet. Assuming that this common energy source provides a constant voltage, the impedance which the electrical conductor heating element has determines the amount of current flowing through the heating element of the electric blanket when the electrical conductor is connected to the electrical outlet. If the impedance of the electrical conductor were to decrease, such as by a short-circuit in the electrical conductor, a greater amount of current would flow therethrough. With the increased current flow, the heating element provided by the electrical conductor would get hotter. If the current were to increase sufficiently, the heating element could become so hot that a fire could be started in the electric blanket. On the other hand, if the impedance of the electrical conductor were to increase, such as by an open circuit formed along the electrical conductor, the outlet voltage would appear at the ends of the open circuit which would be hazardous if the user of the electric blanket were to simultaneously touch both ends of the open circuit. In between these extremes of a short-circuit or an opencircuit in the electrical conductor, the impedance could vary to the extent that the electric blanket would also be considered to be malfunctioning at such varied impedance. Because of the adverse effects which could result from an electrical device which is malfunctioning as a result of the changed impedance of the device, it is important both to detect changes in the impedance of a load which is connected to an energy source and to remove the load from such connection to the energy source when the impedance sufficiently changes.
With respect to the electrical load presented by an electric blanket, I am aware of one apparatus which has been used to detect an anomalous or abnormal impedance of the heating element of the blanket and, in response thereto, to disconnect the electric blanket from its energy source. This prior apparatus of which I am aware includes a first lamp connected in parallel with a portion of the electrical conductor heating element of the electric blanket and also includes a second lamp in parallel with a second portion of the electrical conductor heating element of the electric blanket. Each of these lamps is illuminated in correspondence with changes in the impedances of the respective portions of the electrical conductor. When sufficient current flows through either of the lamps because of the changed impedance, that particular lamp is illuminated. The illumination is detected by a photo-detector element forming another part of the prior apparatus of which I am aware. There is a respective photo-detector element for each lamp so that when either of the photo-detector elements receives light from its corresponding illuminated lamp, it generates an electrical signal which triggers a triac having its terminals connected in electrical parallel with the entire electrical conductor of the electric blanket. When the triac is triggered by a signal from one of the photo-detector elements, it short-circuits the electrical conductor of the electric blanket, thereby allowing a sufficiently large current to flow through the short-circuit so that a fuse in the electric blanket/energy source circuit opens and provides an open circuit.
Although this prior device can detect impedance imbalances in a load and cause the load to be disconnected from its energy source, there is the need for an improved apparatus requiring fewer discrete parts so that the improved apparatus can be more compactly packaged and costs less. Additionally, there is the need for such an improved apparatus to be constructed so that it is highly reliable, both mechanically and electrically, and so that it is not susceptible to being falsely triggered by circuit transients.